Hair follicle development and postnatal cycling. A series of epithelial and mesenchymal signals specify when and where primordial skin appendages, comprising an epithelial placode and dermal condensate, will appear. Additional signals bring about the rapid growth and tissue remodeling that underlie formation of a mature hair follicle, with Shh playing a pivotal role in this process. New findings suggest that Shh is also capable of operating in adult skin, where it may be involved in triggering reentry into the active growth phase of the hair cycle, (anagen).